The implementation of SMLT/SLT between Avaya/Extreme nodes has to be done without LACP.

In meanwhile v10.4.4.023 is GA since Sep 6, 2017 - I'd recommend to upgrade the latest available release on both ends (including the ERS-4800) to see if this issue still exists. If yes, I'd recommend to raise an Servicerequest with Avaya support team.